Modern implantable pacemakers are small, battery-powered electronic devices that are programmed to monitor the activity of the heart to determine when the heart is naturally beating, and provide stimulation pulses to atrial and/or ventricular muscle tissue of a patient's heart when the heart is not naturally beating, thereby maintaining a prescribed heart rhythm or rate. Advantageously, a pacemaker may be implanted in a patient, and coupled to the patient's heart via appropriate pacemaker leads that are also implanted. By implanting the pacemaker and leads, the pacemaker becomes an integral part of the patient, and the patient is able to maintain a substantially normal life style without the bother and worry that typically accompany the use of external (non-implanted), life-sustaining medical devices.
Nearly all implantable pacemakers in use today, as well as other implantable medical devices, can be easily monitored and configured by the attending physician in the physician's office. The process of monitoring and configuring a pacemaker is commonly referred to as "programming" the pacemakers. The programming process uses noninvasive telemetry to extract salient information which is used to customize the operation of the pacemaker to fit the individual needs of the patient. Customization is achieved by adjusting a set of "pacemaker parameters" to values that cause the pacemaker to work in an optimum way for the particular patient within whom the device has been implanted.
An important aid that is useful in monitoring the performance of an implantable pacemaker, and to facilitate the physician's understanding of the pacemaker's programmed operation as it interacts with the patient's natural cardiac activity, is the sensing and recording of various "measured parameters" relating to the pacemaker and/or patient. Unfortunately, the measured parameter information is only available to the physician or other medical personnel through a specialized programming device (typically referred to as the "programmer") which extracts the data via telemetry and displays such data in tabular and graphical formats. Thus, such pacemaker measured data becomes available only during office visits when the pacemaker can be physically interrogated by a programming device and the relevant data extracted. Such interrogation procedures, while extracting a great deal of useful information, cannot be extracted except when a patient is connected to an appropriate programming device.
Recently, a form of remote pacemaker monitoring has also been performed between scheduled office visits by the patient for the purpose of verifying the performance of the patient's implanted pacemaker. The remote pacemaker monitoring is accomplished by utilizing a variety of transtelephonic ECG transmission systems. There are a large number of transtelephonic ECG transmission systems currently available in the market. These transtelephonic ECG transmission systems are receiving widespread use because they are capable of transmitting a patients's ECG signal from his or her home or other remote location via a telephone communication system to a receiver located in a medical office, cardiac diagnostic center, or other designated facility. Advantageously, these transtelephonic ECG transmissions are sent periodically between scheduled office visits and are used to ensure that the pacemaker is performing in a satisfactory manner. If the transtelephonic monitoring detects anomalies in the ECG signal, the physician can schedule an office visit for a more detailed evaluation of the patient, and in order to make a detailed assessment of pacemaker performance, reprogram the pacemaker, or recommend surgical modifications, as required.
Disadvantageously, many transtelephonic ECG transmission systems are not configured to transmit information other than a patient's ECG. Not surprisingly, critical information other than the patient's ECG signal exists, such as the "measured parameters" that relate to the operating status of the pacemaker and/or patient, that is not readily ascertainable with an analysis of the standard ECG signal. An analysis of this other critical information could also prompt the physician to schedule an office visit for a more detailed evaluation of the patient and/or the implanted device. For example, critical patient/pacemaker information typically not available with transtelephonic ECG transmission systems, or through an analysis of the ECG signal, includes information relating to the status of the pacemaker battery, the sensitivity of the pacemaker sensors, the condition of the pacemaker leads, detected conditions relating to the patient/pacemaker such as specified rhythmic conditions, as well as other selected parameters of the implanted pacemaker.
Other related art devices and/or techniques exist for transferring pacemaker-related information from an implanted pacemaker to an external device. The most common technique involves sensing the ECG signal using an external cardiac monitoring device. Like the programming devices, or "programmers", many of these cardiac monitoring devices are designed to perform post-receipt processing and/or analysis of the ECG signal. The post-receipt processing of the ECG signal may include data compression, modulation, A-D conversion, amplification, and other well known ECG signal processing techniques.
One example of a related art system is found in U.S. Pat. No. 5,289,824, issued to Mills et ale, where there is disclosed a compact, lightweight wrist-worn cardiac data and event monitor for the recording and teletransmission of the patient's ECG data together with pacing event data. The disclosed system has prescribed circuitry which includes signal detection; data conversion, storage, display and telephonic transmitter. Event data that may be recorded along with the ECG data include time-of-day, elapsed time markers, and pulse detection markers. Both the event data as well as the ECG data are subsequently processed and analyzed in accordance with prescribed algorithms in the monitoring device.
Another example of a related art cardiac monitoring device is disclosed in U.S. Pat. No. 4,531,527, issued to Reinhold, Jr. et al., which discloses a cardiac monitoring system that detects a patient's ECG and analyzes the ECG in real time (during the patient's R--R interval). The cardiac monitoring system provides morphology analysis, heart rate data, ST segment analysis, symptomatic and asymptomatic event recordings, and the counting of ectopic runs. The analyzed data is sent over a standard voice-grade telephone line or other suitable communication channel to a central location.
Alternatively, many of the related art techniques for passing information from an implanted pacemaker to an external device involve telemetry systems. The telemetered information typically includes intracardiac data, marker data, pacing event data and other pacing related information that originates from the implanted pacemaker and is transmitted to an external device, ire., a "programmer", where the data is further processed and analyzed. Some systems adapted to accept telemetered pacing event records from the implanted pacemaker also include means for forwarding such information via telephone to a remote receiving location.
An example of such related art systems is disclosed in U.S. Pat. No. 4,142,533, issued to Brownlee et al. This patent discloses a complete system for telemetering and monitoring the functioning of an implanted pacemaker. The disclosed system specifically includes the capability for directly and simultaneously transmitting from the pacer, electrical signals indicative of pacer functions, such as, pacer rate, cell voltage, refractory period, heart rate with pacer inhibited, R-wave level and sensing margin, sensing circuit and other component failure, cardiac electrode lead break, and hermetic integrity. The telemetered signals are picked up at the patient's location for local analysis by an external programming device. Such signals, once at the programming device, may be telephonically communicated to a remote central monitoring station. (See also U.S. Pat. No. 4,332,256, issued to Brownlee et al.)
There are few known prior art systems that provide pacemaker pulse modulation, and in particular pulse-interval modulation of the ventricular stimulation pulses in order to convey information concerning the status or condition of critical pacer/patient parameters. One such system is disclosed in U.S. Pat. No. 4,312,354, issued to Walters. The '354 patent discloses an implantable pacemaker having circuit means for indicating, by pulse width modulation of the delivered pacing pulses, the programmable control states which control the selected pacemaker operating parameters. The circuitry disclosed in the '354 patent includes a parallel to serial shift register adapted to receive the control states from a pacer circuit and to generate a serial multiple bit word when the pacemaker is switched to a magnetic mode. The shift register is clocked by stimulus timing signals, and the multiple bit word drives a one shot generator, the output of which is combined with the timing pulses to produce pulse width modulated pacing pulses.
Another system known in the art is disclosed in U.S. Pat. No. 4,151,513, issued to Menken et al., which discloses an apparatus for sensing and transmitting the stimulating pulse of an implanted pacemaker over readily available, low-bandwidth transmission media to a remote receiver capable of measuring the pulse width to provide an indication of the condition of the pacemaker battery. In particular, the sensing and transmitting apparatus detects the electrical activity, e.g. the ECG, of a patient through attached electrodes and modulates a carrier or tone of a suitable frequency to be transmitted over the low-bandwidth transmission medium to the receiver, along with a pulse signal multiplied by a selected, amplification factor dependent upon the carrier frequency of the signal and the other information to be transmitted to the receiver. Typically, the patient in which the pacemaker is implanted has a transmitter for transmitting the aforementioned data to a receiver located in the doctor's office, clinic or hospital, where a regular check of the pacemaker's energy source may be made by the patient's doctor.
Neither of the systems disclosed in the '513 Menken et al. nor the '354 Walters patents provide a simple system for conveying needed information, not related to the ECG, over existing transtelephonic systems. What is needed is a simple and easy to implement technique for transtelephonically transmitting critical pacemaker/patient information to a remote facility for subsequent analysis, thus avoiding the time and expense of performing office-based examinations. The simple technique should be compatible with existing transtelephonic transmission systems, yet be able to convey information not readily ascertainable through standard ECG or intracardiac electrogram analysis.